The present invention relates to a printer of the type using a type wheel, or so-called daisy wheel, and constructed to select a desired type by rotating the type wheel and strike the selected wheel finger by a hammer against a platen to sequentially print out alphanumeric and other characters on a sheet, which is wrapped around the platen. More particularly, the present invention is concerned with a type wheel casette for accommodating the type wheel in a cassette case and a mechanism for removably loading the cassette in a carriage of a printer.
In a type wheel cassette of the kind described, displacement of the type wheel within the cassette case has to be restricted in order to prevent, for example, types from being damaged due to contact thereof with the cassette case when the wheel cassette is handled alone. It is another requisite that the type wheel be always held in a predetermined position inside the cassette case so that when mounted on a carriage of a printer it may be easily connected to a selection motor, which is adapted to rotate the type wheel.
The type wheel described usually includes a hub located at the center of its rotation, a plurality of fingers extending radially outwardly from the hub, and type characters provided in tip portions of the respective fingers. The hub is formed with an opening at its center and an engaging opening outwardly of the central opening. Meanwhile, the cassette case is made up of a front plate and a rear plate and receives the type wheel in a space defined between the front and rear plates. A flat retainer spring is fit on the surface of the front plate to position the type wheel inside the cassette case. The retainer spring is provided with a lug at its intermediate portion and another lug at its end portion.
In the above construction, one has to insert the type wheel into the cassette case, then put the central lug of the retainer spring in the central opening of the type wheel to position the center, and then put the other lug of the retainer spring in the engaging opening of the type wheel. Stated another way, one cannot accomplish accurate positioning of the type wheel in the cassette case merely by inserting the former into the latter, that is, without mating the opening of the type wheel and the lug of the cassette case. The assembly thus requires a troublesome procedure.
Another known approach to position the type wheel in the cassette case is disclosed in, for example, Japanese Patent Laid-Open Publication No. 55-79159. The disclosed approach consists in providing a specially configured projection on a type wheel and specially configured opening in a cassette case, instead of the interengaging openings and lugs described above. Although this approach may successfully restrict the displacement of the type wheel inside the cassette case, it also requires a troublesome assembling procedure as discussed above.
Further, in loading the cassette in the carriage of a printer or a typewriter, it is at least necessary to operatively connect the type wheel to an output shaft of a selection motor which functions to select a desired type on the type wheel and move it to a print position. In this instance, an output shaft of the motor needs to be held in a predetermined angular position before connection to the type wheel. Therefore, an exclusive motor control circuit and other circuits need be built in the body of the printer or the typewriter at the cost of the intricacy of the overall construction and arrangement.
Meanwhile, a printer or a typewriter of the kind using the above-described type wheel cassette comprises a housing which is made up of an upper cover and a lower cover. A platen, a carriage and other mechanisms are installed in the housing. The top of the housing is closed by a transparent top cover which is free to open and close. The drawback particular to such a housing structure is that a wheel cassette cannot be replaced with another unless the top cover is opened and closed each time by awkward manipulations. Besides, since the mechanical arrangements inside the printer become exposed to the outside when the top cover is opened, undesired objects may drop on them to bring about failures.